This invention relates to a method and apparatus for determining heat flux through heated wall members of heat emitting equipment. More particularly, the invention relates to a method and apparatus for controlling the energy balance in a molten salt reduction cell by determining the heat flux through the cell walls and thereafter adjusting the energy added to the cell to control frozen electrolytic lateral ledge thickness.
Control of heat energy may be an important consideration in the proper operation of rotary kilns, furnaces and molten salt reduction cells, for example. Heat balance control can singly or in combination maximize rates of reaction, minimize heat losses and stabilize operation of the device. For example, in aluminum reduction cells, it is desirable to operate a cell with a nearly constant frozen lateral ledge or side crust layer for most efficient operation. Freezing and melting of the side crust due to changes in the cell heat balance can theoretically be kept within small limits by adjusting the cell energy input based on crust thickness measurements. However, the side crust generally cannot be measured directly on a regular basis for all cells.
It has been suggested that the thickness of the lateral ledges of frozen electrolyte in a cell for the electrolytic recovery of aluminum can be controlled by using the changes of level of the anode. U.S. Pat. No. 3,900,371, issued Aug. 19, 1975, discloses a method of moving the anode beam to minimize a difference between the measured instantaneous ohmic cell resistance and the base resistance. It has also been suggested that the energy balance in an aluminum reduction cell can be controlled by measuring temperature in the side cathodic lining of the cell and comparing the measured temperature with a reference temperature. U.S. Pat. No. 4,045,309, issued Aug. 30, 1977, discloses such a method whereafter the immersion level of the anodes is adjusted within the electrolyte. The method involves a certain time lag which permits the method to be useful for slow disturbances in the heat balance; however, the system will be unable to correct faster changes.
The interrelationship of many factors of a reduction cell are the subject of an article entitled "Variations of Side Lining Temperature, Anode Position and Current/Voltage Load in Aluminum Reduction Cells", by Paulsen et al, Light Metals, 1980, which reports an investigation concerning the dynamic behavior of the side ledge thickness and the combined data of temperature, anode position and current/voltage to give information suited for computer control of the cell.
Calculated and measured values of heat fluxes can be used in determining the thickness of frozen ledges within a molten salt reduction cell, as shown in the article entitled "Calculating Thickness of Containing Walls Frozen from Melt", by W. E. Haupin, Journal of Metals, July 1971. A rise in heat flux indicates a thinning of the ledge and a lowering in heat flux indicates a thickening of the ledge. Melting of the ledge absorbs heat and freezing of the ledge gives up heat tending to hold the electrolytic bath temperature constant. Under aggressive conditions in a molten salt reduction cell, for example, it is sometimes impractical to measure the temperature or heat flux within a furnace or reduction cell. Measurement of heat flux through the wall, however, appears to be more sensitive to changes in heat balance than measurements of absolute temperature within the electrolyte and offers an opportunity to give better control to aluminum reduction cells. The use of heat flow transducers or sensors to measure heat passing through the walls of the cell can aid the control of heat input and maintain a more constant balance of the cell.
Heat flow sensors to determine heat flux, essentially in one dimension along the axis of a probe and thereby control heat input into an apparatus to maintain heat balance are known. U.S. Pat. No. 3,267,726, issued Aug. 23, 1966, relates to a probe arranged on a longitudinal axis having a probe member and a support member attached as by welding to the outer surface of the wall through which the heat flux is to be determined. Arranged in the probe are thermocouple junctions 26 and 23 which provide the heat transfer measuring function between the hot junction 26 extending to the interior surface of the wall and the junction 23 located outside the wall. U.S. Pat. No. 3,437,325, issued Apr. 8, 1969, similarly discloses a control apparatus for a rotary kiln by measuring the heat loss through walls of the kiln using differential thermocouple locations with innermost thermocouple 88' and outermost thermocouple 86'.
Such heat flow sensors are more complicated than thermocouple devices which measure temperature. U.S. Pat. No. 3,016,412, issued Jan. 9, 1962, discloses a thermocouple device having an elongated tubular body 1 of refractory or high heat resistant material such as stainless steel or Inconel having mounted therein a pair of thermocouple lead wires forming a bimetallic junction. U.S. Pat. No. 3,503,260, issued Mar. 31, 1970, relates to a thermocouple pyrometer using thermocouple leads of constantan and chromel and in which the thermocouples can be serially connected.
What is needed, however, is a method and apparatus for measuring heat flux from the wall of a furnace or reduction cell which is inexpensive and less complicated than prior art devices, yet takes advantage of the heat flux measurement as being more sensitive to changes in heat balance than temperature in order to give a better control of the reduction cell. The apparatus should include an inexpensive heat flow sensor that outputs a relatively large signal but that is very small and does not interrupt or affect the heat flow from the surface being measured. Furthermore, the apparatus should be capable of operating at surface temperatures up to 800.degree. C. and it should be able to be quickly installed on hot wall surfaces of the cell or furnace without the need to shut down the cell or furnace. The method and apparatus should also have the capability of measuring surface temperature as well as the heat flow from the wall member being measured.